This invention relates generally to packaging and specifically to packaging laminates for food products.
It is common practice to package articles such as food products in thermoplastic films or laminates to protect the product to be packaged from abuse and exterior contamination, and to provide a convenient and durable package for transportation and sale to the end user.
Typical thermoplastic packaging films and laminates which provide the desired abuse protection and other desirable features in the finished package also tend to form strong heat seals when sealed together about the product. These materials, after heat sealing, are often difficult to open by the consumer or end user. It has therefore been desirable to have a packaging material which provides adequate protection of the product against environmental contaminants, moisture and air, and sufficiently strong heat seals to maintain the integrity of the package through the distribution channels and storage, and at the same time provide a heat seal which is easily opened i.e. easily peelable by the consumer or end user. Such an easy open feature avoids the need to tear apart the package with knife or other implements when it is desired to use the product.
It is also desirable to provide packaging materials with the above benefits and which in addition have bonding capabilities, so that upon packaging a product such as a food product, for example in a thermoforming process, an upper and lower laminate will bond in contiguous areas around and up to the product, providing a sufficiently strong bond between the laminates to maintain the integrity of the package and protection of the packaged product through the distribution cycle.
Another feature of a packaging system, which would be of great benefit to the food packaging industry, is the capability of providing approximately the same ease of openability under a wide range of sealing conditions. Currently, most easy-open systems will exhibit an easily peelable seal when an article is placed in the thermoformed web in a particular way. However, with variations in product placement and size, the bonded area of the final package can become greater. This in turn means that more work must be accomplished to open a particular package if the peel force remains the same. Ideally, an easy-open feature will permit packages to be opened with a sufficiently low, opening force to permit easy opening of the package regardless of product placement.
Of interest is U.S. Pat. No. 3,655,503 which issued to Hugh E. Stanley et al on Apr. 11, 1972. This reference shows a package of a composite, heat sealable thermoplastic film having an outer and inner layer with a relatively low interlayer bond strength. This film is used by overwrapping about the article to be packaged; the film is then heat-sealed to itself at overlapping seams. Ionomers are suitable resins for the outer layer and inner layer. The ionomer when used as an inner layer is used in conjunction with polypropylene. The present invention, in contrast, uses two distinct webs to package an article by thermoforming or other suitable means. The Stanley patent teaches an easily opened heat seal in which the heat seal zone comprising bonded outer layers of the overlapped film is first ruptured during opening, and then delamination starts along the interface between an outer and inner layer. At the end of the heat seal zone, the outer layer is once again ruptured to complete the peeling of the package. Rupturing and delamination may occur more than once throughout the heat seal zone during opening. The reference shows only a single resin used in the outer or an inner layer.
Also of interest is U.S. Pat. No. 4,178,401 issued to Alan S. Weinberg et al. This reference discloses a self-welding packaging film in which the inner or self-welding layer is a blend of ethylene vinyl acetate copolymers with differing melt flow indices. Blends of ionomer of low melt flow with ethylene vinyl acetate copolymers with high melt flow are also disclosed, but these latter blends show poor self-eldability. An easily peelable heat seal is not shown.
Also of interest is U.S. Pat. No. 4,382,513 issued to Henry G. Schirmer et al on May 10, 1983. This patent discloses a package having a peelable heat seal wherein unoriented films are irradiated to control the bond strength of the resulting heat seal. Ionomers of ethylene and ethylene vinyl acetate copolymer are possible sealing layers of thermoplastic films used with this irradiated process.
It is an object of the present invention to provide a package which is made from materials including sealing layers of incompatible materials, thereby providing an easily opened seal.
It is a further object of the present invention to provide a package for food products which includes a first web and a second web which bond to each other along their contiguous surfaces during a thermoforming process.
It is a further object of the present invention to provide a package for food products which has good bonding characteristics and good seal strength, yet is easily peelable through both the bonded areas and fusion seal areas of the package.
It is also an object of the present invention to provide a heatsealed package which can be easily opened with a relatively consistent, low seal opening force, over a relatively wide range of sealing conditions.